In an optical communication field or an optical measurement field, various kinds of optical control devices, such as a waveguide type optical modulator or a waveguide type optical switch, in which an optical waveguide or a control electrode is formed on a substrate having an electro-optical effect have been often used in the related art.
In many types of optical control devices currently used, an optical waveguide 2 or a signal electrode 4 and a ground electrode 5 are formed in an electro-optical crystal substrate 1 with a thickness of about 0.5 to 1 mm as shown in FIG. 1A. In addition, FIG. 1A is an example of an optical modulator using a Z-cut type LiNbO3 substrate, and reference numeral 3 denotes a buffer layer, such as an SiO2 film.
Particularly in the waveguide type optical modulator, a microwave signal is applied to a control electrode in order to control modulation of a light wave propagating through an optical waveguide. Accordingly, in order for a microwave to efficiently propagate through a control electrode, impedance matching between of a signal line, such as a coaxial cable, through which a microwave is introduced into the optical modulator, and the control electrode in the optical modulator is required.
For this reason, as shown in FIG. 1A, a shape in which the signal electrode 4 is interposed between the ground electrodes 5, a so-called coplanar type control electrode is used.
In the case of the coplanar type control electrode, however, an external electric field does not act efficiently in the direction (vertical direction in the case of the Z-cut type LiNbO3 substrate shown in FIG. 1A) where the efficiency of the electro-optical effect of the substrate 1 is high. Accordingly, a larger voltage is needed in order to obtain a required degree of optical modulation. Specifically, in the case where an LiNbO3 (hereinafter, referred to as ‘LN’) substrate is used and the electrode length along the optical waveguide is 1 cm, a half-wave voltage of about 10 to 15 V is required.
In addition, as shown in FIG. 1B, a configuration in which an optical waveguide is made as a ridge type waveguide 20 and ground electrodes 5, 51, and 52 are disposed closer to electrodes 4 and 41 in order to improve confinement of a light wave of the optical waveguide and apply an electric field, which is generated by a control electrode, to the optical waveguide more efficiently is proposed in Patent Document 1. By this configuration, a reduction of a certain amount of driving voltage can be realized. However, in order to realize high speed modulation particularly in a high frequency band, a much more driving voltage should be reduced.    [Patent Document 1] U.S. Pat. No. 6,580,843
Furthermore, as shown in FIG. 1C, interposing a substrate between control electrodes and applying an electric field in the direction (vertical direction in the case of a Z-cut type LiNbO3 substrate shown in FIG. 1C) where the efficiency of the electro-optical effect is high are proposed in Patent Document 2. In addition, in an optical modulator shown in FIG. 1C, a substrate having an electro-optical effect is polarization inverted, substrate regions 10 and 11 where directions (directions indicated by arrows in the drawing) of spontaneous polarization are different are formed, and an optical waveguide 2 is formed in each substrate region. Accordingly, in the case when an electric field is applied to each optical waveguide with common signal electrode 42 and ground electrode 53, it becomes possible to cause a phase change in opposite directions to occur in light waves propagating through respective optical waveguides. By such differential driving, it becomes possible to further reduce a driving voltage.    [Patent Document 2] Japanese examined Patent Publication No. 3638300
However, in the electrode structure shown in FIG. 10, the refractive index of a microwave increases and accordingly, speed matching between a light wave propagating through the optical waveguide and a microwave which is a modulated signal becomes difficult. In addition, since impedance decreases conversely, there is a disadvantage that impedance matching with a signal line of a microwave becomes also difficult.
On the other hand, In the following Patent Document 3 or 4, an effective refractive index of a microwave is reduced by providing an optical waveguide and modulation electrodes on a very thin substrate (hereinafter, referred to as a ‘thin plate’) having a thickness of 30 μm or less and bonding another substrate having a dielectric constant lower than the thin plate to the thin plate, such that the speed matching between the microwave and the light wave is realized.    [Patent Document 3] JP-A-64-18121    [Patent Document 4] JP-A-2003-215519
However, even in the cases where the control electrodes with the structures shown in FIG. 1A to 1C are formed for the optical modulator using such a thin plate, the above-described problems are not solved yet basically. In the case where the substrate is interposed between the control electrodes as shown in FIG. 1C, if the thickness of the substrate is made small, it is difficult to realize speed matching between a light wave and a microwave even though the microwave refractive index tends to decrease. Although it also depends on the width of an electrode, the effective refractive index is about 5 in the case of using a thin plate of LN, for example, which does not approximate 2.14 that is an optimum value. On the other hand, the impedance tends to decrease as the substrate becomes thin, which becomes a cause of increasing impedance mismatch.